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        // 预解析案例
        // 案例1
        var num = 10;
        fun();
        function fun(){
            console.log(num );
            var num=20;
        }
        // 相当于执行了以下代码
        var num;
        function fun(){
            var num;
            console.log(num);
            num=10;
        }
        num=10;
        fun();

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